Microsoft's Cortana voice assistant will be launched as an app for iPhone and Android, Microsoft has announced. Although the rival OSes already have their own voice assistants - Siri and Google Now respectively - Microsoft's hoping to muscle them aside with the system that's also being built into Windows 10 .

"Part of the power of a personal assistant comes from being available on the go, on the device you carry with you everywhere," writes Microsoft's corporate vice president, Joe Belfiore on the Windows blog . "And for people who don’t have the benefit of a Windows phone, we want to extend the advantage of Cortana in Windows 10."

Microsoft says the iOS and Android versions of Cortana will be able to do "most" of the things than Windows Phone version can, such as setting reminders and alarm calls, tracking flights and delivering notifications for appointments, as well as answering voice search queries. However, the company warns that because Cortana isn't directly embedded into iOS or Android, it will lack some of the features it can deploy on Windows Phones.

" Some features require access to the system that aren’t currently possible with iOS or Android, so things like toggling settings or opening apps won’t initially be available in the Cortana companions for those platforms," says Belfiore. "Similarly, the ability to invoke Cortana hands-free by saying 'Hey Cortana' requires special integration with the device’s microphone, so that feature will be limited to Windows Phones and PCs."

Microsoft already has a voice assistant in the Google Play Store called Bing Torque, which offers many of the same features as Cortana. It's activated by shaking the handset, instead of by voice command.

Cortana can be installed on iPhones and Android handsets via a new Phone Companion app that's being built into Windows 10. This will direct users to install various Microsoft apps on their handsets, including OneDrive, OneNote and the various Office apps. Microsoft will also offer an Xbox Music app on the rival platforms, which will allow users to stream any music stored in their OneDrive on their mobile phone for free.

The Cortana app won't be released on Android until the end of June and on iPhone "later in the year". The Phone Companion will appear in a fresh build of the Windows 10 Preview within "a few weeks".